1. Field of the Invention
This invention relates to liquid crystal color display cells and more particularly to a nematic type liquid crystal color display cell utilizing a Guest-host effect.
2. Description of the Prior Art
As a color display process which makes use of a nematic liquid crystal, both a process utilizing an Electrically controlled birefringence effect (ECB effect) or Deformation of vertical aligned phases effect (DAP effect) and a process utilizing the Guest-host effect have heretofore been well known.
In a liquid crystal color display cell utilizing the ECB effect, if the liquid crystal has a positive dielectric anisotropic property, the liquid crystal is sandwiched between two substrates which are treated such that a long axis of a liquid crystal molecule is homogeneously aligned with a horizontal direction between these two substrates. If the liquid crystal has a negative dielectric anisotropic property, the liquid crystal is sandwiched between two substrates which are treated such that the long axis of the liquid crystal molecule is homeotropically aligned with a vertical direction between these two substrates. A voltage is applied to the liquid crystal which is higher than a threshold voltage sufficient to incline the liquid crystal molecule with respect to the substrate. As a result, the alignment of the liquid crystal molecule when the voltage is not applied thereto is inclined with respect to the substrate so as to change the double refraction property of a crystal layer. In this way, the liquid crystal color display cell utilizing the ECB effect functions to effect the color display by means of two polarization plates crossed perpendicularly with each other.
The liquid color display cell utilizing the ECB effect has a number of disadvantages. In the first place, the threshold voltage is so high that the operating voltage becomes high. Secondly, a range of the applied voltage within which the color changes yell.fwdarw.orange.fwdarw.red.fwdarw.purple.fwdarw.blue in response to the degree of inclination of the liquid crystal molecule is very narrow, so that a slight voltage change causes the color phase to change. Thirdly, the visual angle is extremely narrow and hence it is necessary to use a projector type cell, thereby significantly restricting application fields. Finally, if the two substrates are not uniform in thickness, such non-uniform thickness of the substrates results in an unevenness of the display color, so that in order to provide a cell which can precisely effect the color display, its manufacture becomes difficult.
As described above, the liquid crystal color display cell utilizing the ECB effect has the number of disadvantages, so that at present such cell is not used as an efficient liquid crystal color display cell. In the liquid crystal color display cell utilizing the Guest-host effect, if a very small amount of a dichroic coloring agent having a bar-shaped molecular structure and constituting a "guest" is added to a nematic liquid crystal for constituting a host, the liquid crystal molecule becomes changed in orientation in response to a voltage applied to the liquid crystal so as to cause the guest molecule to change its orientation, thereby making the attraction properties of the liquid crystal molecules different from each other. This difference between attraction properties of the liquid crystal molecules is utilized to effect the color display.
The liquid crystal color display cell utilizing the Guest-host effect has the advantages that the visual angle is made larger and the transmittivity is made higher than those of the other liquid crystal color display cell, and that the color phase and coloring degree can be adjusted at will by suitably selecting the guest.
As described above, the dichroic coloring agent to be added as the guest of the Guest-host effect has the bar-shaped structure. In general, the coloring agent having the bar-shaped structure has a feature that if the direction of the long axis of the molecule thereof coincides with the direction of an electric field vector of an incident light, a light absorbing power of the coloring agent becomes maximum and the transmittivity becomes minimum and that if the direction of the long axis of the molecule thereof becomes perpendicular to the direction of the electric field vector of the incident light, the color absorbing power of the coloring agent becomes minimum and the transmittivity becomes maximum. As a result, in the liquid crystal color display cell utilizing the Guest-host effect, in order to change the color absorbing power of the coloring agent as large as possible, provision may be made of the following two liquid crystal color display cells.
In the former cell, between two substrates which are treated such that the liquid crystal molecules are homogeneously aligned is sandwiched a liquid crystal including a "host" formed of a liquid crystal having a positive electric anisotropic property and added with a very small amount of a guest and to the liquid crystal is applied a voltage so as to align the liquid crystal molecule perpendicular to the substrate plane.
In the latter cell, between two substrates which are treated such that the liquid crystal molecules are homeotropically aligned is sandwiched a liquid crystal including a host formed of a liquid crystal having a negative dielectric anisotropic property and added with a very small amount of guest and to the liquid crystal is applied a voltage so as to orient the liquid crystal molecule horizontally with respect to the substrate plane.
The former liquid crystal color display cell can change over the color display from a color display to a colorless display in response to the voltage applied thereto, while the latter liquid crystal color display cell can change over the color display from the colorless display to the color display in response to the voltage applied thereto. As a result, the latter liquid crystal color display cell utilizing the Guest-host effect provides a display part which can exhibit a color phase of a dichroic coloring agent for changing over the color display from the colorless display to the color display in response to the voltage applied thereto and hence is more suitable as the display system than the former cell. In the liquid crystal color display cell utilizing the Guest-host effect and comprising two substrates which are pretreated such that the liquid crystal molecules are homeotropically aligned, if the homeotropical aligning treatment causes the liquid crystal molecule to incline at an angle within a range between an angle smaller than 90.degree. and an angle of several degrees to a normal line drawn perpendicular to the substrate plane and if this direction of inclination is not aligned with one direction, the liquid crystal molecules are inclined in different directions in the case of changing the orientation of liquid crystal molecules into the horizontal direction to the substrate plane in response to the voltage applied thereto as shown in FIG. 1, thereby deteriorating contrast and inducing unevenness in color.
In a liquid crystal color display cell which can eliminate the above mentioned drawback by orienting the liquid crystal molecules from the condition shown in FIG. 2 in which the liquid crystal molecules are aligned in one direction and inclined at an angle of substantially right angles to the substrate plane to the condition shown in FIG. 3 in which the liquid crystal molecules are aligned in one direction and alinged with a horizontal direction between the substrates by applying the voltage thereto, the liquid crystal molecules are aligned with one direction and also aligned with one horizontal direction between the substrates in the case of applying the voltage thereto, so that the maximum absorption is effected to only the incident light whose electric field vector coincides with the long axis direction of the guest molecule added to the liquid crystal.
That is, in FIG. 4, the maximum absorption is effected for an incident light 5 whose electric field vector is perpendicular to an X axis, and as a result, in the case of viewing the liquid crystal color display cell from directions other than the direction of the incident light 5, the maximum absorption is not obtained and the absorption factor becomes changed in dependence with the angle of viewing the cell. As a result, the liquid crystal color display cell utilizing the Guest-host effect has the advantage that the visual angle is wider than that of the other liquid crystal color display cell, but has the disadvantage that the cell could not be viewed with the same color strength irrespective of the view angles.
In addition, the conventional liquid crystal color display cell utilizing the Guest-host effect can effect the color display, but the change in the color degree is restricted to the single color phase only, so that the multi-colors display effected by the liquid crystal color display device utilizing the ECB effect could not be obtained. That is, the liquid crystal color display cell utilizing the Guest-host effect and comprising the two substrates treated to be homeotropically aligned functions to change the color display from the colorless display to the color display in response to the voltage applied thereto and is adapted to the display system. In this cell, the color degree change of the single color is called as the color display, but does not means the color display of different color phases.